The present invention relates to a polyester resin-covered metal sheet possessing excellent formability. The covered sheet is particularly suited for xe2x80x9cheavily formedxe2x80x9d use, such as drawing, drawing and ironing, drawing and stretch forming, and ironing after drawing and stretch forming. The present invention also teaches a method for forming said resin-covered metal sheet.
Metal containers such as beverage cans or battery containers are typically formed by drawing, drawing and ironing, drawing and stretch forming, or ironing after drawing and stretch forming. These drawing processes expand the interior volume of the metal container by reducing the thickness of the surrounding walls. Subsequent to drawing, the containers are usually laminated with a corrosion resistant coating and printed with desired text and indicia.
This process may be enhanced by first coating the metal sheets with an organic resin, such as polyethylene terephthalate (PET). Initial coating of the subsequently-drawn metal reduces coating costs and mitigates environmental pollution resulting from dispersion of solvents during the application of corrosion resistant coatings. Such resin-coated metal cans have already been utilized in beverage cans.
Suitable organic resins, in the form of a biaxially-oriented film, are heat-bonded to metal sheets prior to the drawing process. These films are manufactured via biaxial elongation of a thermoplastic polyester resin, followed by heat-setting. Their mechanical characteristics, when measured with a tensile tester, are characterized by large yield strength and small elongation (elongation after fracture).
Alternatively, the resin films may be laminated onto a metal sheet with adhesive, so as to avoid the loss of biaxial orientation that results from heat-bonding. However, due to their limited ability to elongate, these resin coatings exhibit numerous fractures and cracks. Furthermore, where there is only limited adhesion between the metal and the resin, the resin tends to peel off during the lamination process. Furthermore, where heat bonding is utilized to laminate the polyester resin onto the metal sheet, the biaxial orientation of the resin film is partially or entirely lost. Consequently, the yield strength of the post-lamination resin film decreases while elongation improves, preventing the film from cracking, peeling-off, or fracturing. Conversely, resin films lacking biaxial orientation have such large permeability that the contents of a container laminated therewith permeate the film and corrode the metal substratum. Such films also tend to generate coarse spherlites during the printing process, and tend to crack readily if containers collide or fall.
In biaxially oriented polyester resin films heat bonded to metal sheets, the elongation after fracture, prior to lamination, is defined in one of the following ways:
1. According to the preferable range described in Laid open Japanese patent Hei 1-249331,
2. As the range of the orientation coefficient showing the degree of biaxial orientation prior to lamination,
3. As the preferable range of elongation after the fracture and tensile strength are defined, as illustrated in Laid open Japanese patent Hei 2-70430.
Processes like those disclosed in Laid open Japanese patents Hei 1-249331 and Hei 2-70430, which utilize heat bonding to laminate a biaxially oriented resin film onto a metal sheet, effectively destroy the resin""s biaxial orientation. This alters the values of the post-fracture elongation and the tensile strength. Thus, previously acceptable, biaxially oriented films, subsequent to heat lamination, may no longer exhibit the same favorable biaxial orientation; the film""s favorable elongation and tensile strength will also be compromised.
It is an object of the present invention to overcome the deficiencies in the prior art. Accordingly, the present invention produces a polyester resin film-covered metal sheet
The values of the elongation after fracture (disclosed in Laid open Japanese patent Hei 1-249331) and elongation after fracture and disclosure of tensile strength (Laid open Japanese patent Hei 2-70430) are determined prior to lamination of the resin to the metal sheet. characterized by excellent formability and adapted for use in conventional drawing, drawing and ironing, drawing and stretch forming, and ironing after drawing and stretch forming processes.
According to the present invention, a polyester resin film-covered metal sheet, which retains the biaxial orientation of the resin subsequent to lamination, has a true stress value ranging from 3.0 to 15.0 kg/mm2 measured at 75xc2x0 C. and corresponding to a true strain of 1.0. In a preferred embodiment, the polyester resin is a polyethylene terephthalate resin having a low crystallization temperature, (i.e., the temperature of the exothermic peak produced upon heating a quenched sample of the resin in a differential scanning calorimeter) between 130 and 165xc2x0 C., preferably 140 to 155xc2x0 C. The polyester resin is preferably a copolyester resin of recurring ethylene terephthalate or butylene terephthalate monomers. Alternatively, it may be a copolyester resin consisting of at least two of the ethylene or butylene terephthalate monomers, or a double layered polyester resin consisting of a laminate of at least two of the afore-mentioned resins.
The method of producing the present invention entails contacting the polyester resin to a metal sheet, heating the composite to a temperature above the melting temperature of the polyester resin, and pinching and pressing the composite into a laminate with a pair of laminating rolls. The laminating rolls form a nip at the exit site of the laminate, said nip being equipped to cool the emerging laminate at a rate of at least 600xc2x0 C./second. The resulting laminate exhibits a true stress of 3.0 to 15.0 kg/mm2 measured at 75xc2x0 C. and corresponding to a true strain of 1.0.
The resin contemplated for use in the present invention consists of a polyester resin, preferably polyethylene terephthalate, having a low crystallization temperature ranging from 130 to 165xc2x0 C., optimally between 140 to 155xc2x0 C. Alternatively, the resin may constitute a copolymer of ethylene terepthalate and ethylene isophthalate monomers. Either formulation, when applied to a metal sheet according to the present invention, results in a laminate of decreased yield strength and increased elongation, thereby reducing occurrences of film peeling, cracking, or fracture.
The present invention relates to the coating of metal sheets with a polyester resin film, preferably polyethylene terephthalate (PET), biaxially oriented along its length and width, and having a low crystallization temperature ranging from 130 to 165xc2x0 C., optimally between 140 and 155xc2x0 C. This latter value will be explained subsequently.
When an amorphous polyester resin, such as PET, is obtained by heating said resin above its melting temperature, immediately quenching the resin, and then gradually heating with a differential scanning calorimeter (DSC). This process generates an exothermic peak between temperatures of 100 and 200xc2x0 C., depending upon the resin composition. Resins with exothermic peaks at higher temperatures exhibit lower crystallization velocities compared with those characterized by lower-temperature exothermic peaks. For example, polybutylene terephthalate resins produce an exothermic peak at about 50xc2x0 C., whereas PET generates a peak at about 128xc2x0 C. In contrast, an ethylene terephthalate -ethylene isophthalate copolyester resin (typically used in xe2x80x9c2-partxe2x80x9d cans) exhibits an exothermic peak at about 177xc2x0 C.
According to the present invention, a biaxially oriented film of PET resin having a low crystallization temperature outside the 130 to 165xc2x0 C. range can be heat bonded to a metal sheet. However, a PET resin exhibiting a crystallization temperature between 130 and 165xc2x0 C. is better suited to produce a metal-resin laminate that retains the biaxial orientation of the resin, as well as impermability and impact resistance.
Any resin films of PET, polybutylene terephthalate, ethylene terephthalate copolyesters, ethylene isophthalate copolyesters, blended polyesters of two or more of the preceding components, or multilayers of these resins are applicable to the present invention. Where superior impact resistance is required, a bis-phenol A polycarbonate resin may be added to the polyester resin. Alternatively, the bis-phenol A polycarbonate resin or the bis-phenol A polycarbonate resin in combination with the aforementioned resins of the present invention, may be incorporated into the center of into a multilayer resin, the outermost layers consisting of the aforementioned resins of the present invention. Colored resins may be produced by adding pigments to the molten resin during its manufacture.
The thickness of the polyester resin film ranges between 5 and 50 xcexcm, preferably 10 to 30 xcexcm. Thinner films tend to wrinkle and fail to provide uniform coverage of the metal sheet. Films exceeding thicknesses of 50 xcexcm are unnecessary and economically inefficient.
It is essential to the present invention that the resin employed therein has a true stress of 3.0 to 15.0 kg/mm2 at 75xc2x0 C., corresponding to a true strain of 1.0. Metal sheets covered with such polyesters withstand xe2x80x9csevere formingxe2x80x9d methods such as drawing, drawing and ironing, drawing and stretch forming, and ironing after drawing and stretch forming. These drawing methods are carried out at temperatures exceeding the glass transition temperature of the polyester resin, enhancing the formability thereof. The precise technique for heat bonding a polyester resin film to a metal sheet will be described subsequently.
The true strain and true stress of the disclosed polyester resins are measured according to the following procedure:
A resin covered metal sheet is immersed in hydrochloric acid solution, dissolving the metal sheet so that only the polyester resin film remains. A test piece of this film, measuring approximately 5 mm in width and 50-60 mm in length, is subjected to a tensile tester at a temperature of 75xc2x0 C. A cross head distance of 20 mm is maintained. From these parameters, the nominal stress, "sgr"0, and the elongation of the resin, E1, are calculated according to the following formula:
E1=100xc3x97(Lxe2x88x92L0)
wherein
L0: the length of a test piece before stressing
L: the length of a test piece after stressing true strain, xcex5a and true stress "sgr"a, are calculated as follows:
xcex5a=xcex5/(1+xcex5)
"sgr"a="sgr"0/(1+xcex5)
wherein
xcex5: strain
xcex5: E1/100.
The value of true stress, corresponding to the true strain of 1.0, may be obtained from the graph of the true strain-true stress curve.
The resin of the present invention preferably exhibits a true stress value between 3.0 and 15.0 kg/mm2. Resins with lesser true stress values produce uneven coverage of metal sheets, due to the large coefficient of friction that develops between the resin and the laminating machinery. Additionally, these resins lack the impermeability to insulate the metal from corrosion. Resins characterized by a true stress value in excess of 15.0 kg/mm2 tend to crack extensively during lamination, again resulting in uneven coverage of the metal sheet.
In cases where there the resin does not sufficiently adhere to the metal sheet, or where single-ply lamination fails to provide adequate corrosion protection and impact resistance, a thermosetting adhesive (e.g., phenol-epoxy adhesive) is coated on a surface of the metal sheet and dried. Alternatively, the polyester resin film, prior to lamination, may be coated with the thermosetting adhesive. Application of the adhesive, however, often proves expensive, and the solvents used therefor are often detrimental to the environment. It is preferred, then, that the additional step of coating the resin or metal with adhesive be avoided whenever possible.
The metal sheets contemplated by the present invention include surface treated strips or sheets of steel or aluminum alloy. If steel is used, low carbon, tin-free steel, having a thickness between 0.15 and 0.30 mm, is preferred. A two layered coating of metallic chromium (bottom layer) and hydrated chromium oxide (upper layer) is applied to the steel sheet to facilitate subsequent adhesion of the polyester resin. The chromium-chromium oxide coating may also be applied to steel sheets plated with tin, nickel or aluminum, a double layered plating or alloy plating of more than one of tin, nickel or aluminum.
If aluminum alloy is used as the metal sheet in the present invention, the JIS 3000 or 5000 series are preferred because of their economy and formability. It is also preferred that the aluminum sheets be subjected to conventional surface treatments, such as electrolysis, dipping in chromic acid solution, etching in alkaline or acidic solution, or anodic oxidation.
As with the steel sheets, it is also possible to apply a dual-layer, chromium-chromium oxide coating to the surface of the aluminum sheets. In either case, the coating weight of the hydrated chromium oxide is preferably between 3 and 50 mg/m2, (inclusive) chromium, optimally between 7 and 25 mg/m2 (inclusive). The coating weight of the metallic chromium layer ranges from 10 to 200 mg/m2, preferably 30 to 100 mg/m2.
Once the metal sheet has been prepared and coated, the polyester resin film is applied according to the following procedure:
1. A continuous supply of metal is heated to a temperature exceeding the melting point of the polyester resin;
2. A continuous supply of biaxially oriented polyester resin film is contacted with the heated metal strip. The resin film may be applied to one or both sides of the metal strip. Optionally, a thermosetting resin (e.g., epoxy resin) can be inserted between the metal sheet and the resin film;
3. The resin film and heated metal strip are then pressed between two laminating rolls forming a nip at the exit site thereof;
4. The laminating rolls pinch and press the film and the metal strip to ensure adhesion;
5. The resin-metal laminate is then cooled at a rate of 600xc2x0 C./second as it emerges from the nips of the laminating rolls.
In the above process, the heated metal sheets conduct sufficient heat to melt the polyester resin film thereto. The resin loses a greater degree of biaxial orientation nearer its point of contact with the heated metal strip. Biaxial orientation is retained to a greater degree nearer the uppermost surface of the film, farthest from the heated metal strip and closest to the cooling action of the nips of the laminating rolls. Retention of the resin film""s biaxial orientation subsequent to lamination is maximized by controlling the cooling rate of the laminate immediately following lamination. The cooling rate is determined by the temperature of the heated metal strip and the laminating rolls, by the size of contact area between the resin-metal sheet composite and the laminating roll, and by the duration of contact between the metal strips and the laminating rolls. The latter value corresponds with the feed rate of the metal strip and laminating rolls. Generally, greater losses of biaxial orientation occur when the resin film is subjected to excessive heat, such as when the metal strip and laminating rolls reach high temperatures or when the feed rate of the metal strip is relatively rapid and the nip length (determined by the diameter of the laminating roll and the elasticity modulus of the roll) is relatively short.
Optimal retention of desired biaxial orientation, wherein when the resin loses a greater degree of biaxial orientation nearer its point of contact with the heated metal strip and retains a greater degree of said orientation nearer the uppermost surface of the film, farthest from the heated metal strip and closest to the cooling action of the nips of the laminating rolls, is achieved by controlling the rate at which the laminate is cooled as it emerges from the nips of the laminating rolls. A cooling rate of 600xc2x0 C./second appears to result in maximal retention of the resin film""s biaxial orientation. Slower cooling rates do not prevent excessive heating of the resin, resulting in greater loss of orientation. This enhances formability, but reduces impact resistance when the laminate is heated subsequent to forming.